(a) Field of the Invention
The present invention relates to a vertical cavity surface emitting semiconductor laser device (VCSEL device) and, more particularly, to a VCSEL device for use as a light source in the field of optical data transmission and optical communications.
(b) Description of the Related Art
The VCSEL devices have attracted attentions in the data transmission field. The VCSEL device includes a pair of semiconductor multi-layer reflectors (referred to as DBRs hereinafter) overlying a semiconductor substrate, and a resonant cavity sandwiched between the DBRs and including an active layer for laser emission and a pair of cladding layers sandwiching therebetween the active layer. Each DBR includes a plurality of Al(Ga)As/(Al)GaAs layer pairs, and one of the DBRs passes therethrough a laser beam in the direction perpendicular to the substrate surface.
FIG. 11 shows a conventional VCSEL device, which includes an n-type GaAs (n-GaAs) substrate 111, and a layer structure formed thereon by an epitaxial growth process. The layer structure includes consecutively, as viewed from the n-GaAs substrate 111, an n-type lower DBR 112, an n-type lower cladding layer 113, an active layer or active layer structure 114, a p-type lower cladding layer 115, a current confinement layer 116, a p-type upper DBR 117, and a p-type GaAs cap layer 118. The VCSEL device further includes a p-side electrode 119 formed on the layer structure and an n-side electrode 120 formed on the bottom surface of the n-GaAs substrate 111. The active layer (or active layer structure) 114 emits light by recombination of positive holes and electrons injected from the p-side electrode 19 and the n-side electrode 20, respectively.
FIG. 12 shows the structure of the n-type DBR 112 as well as the p-type DBR 117. Each of the lower and upper DBRs 112 and 117 includes a plurality of layer pairs each including an Alx11Ga1-x11As high-reflectivity layer 112A and an Alx12Ga1-x12As low-reflectivity layer 112B, and an Alx13Ga1-x13As slope content layer 112C sandwiched between each of the high-reflectivity layer 112A and each adjacent low-reflectivity layer 112B, wherein 0≦x11<1, 0<x12≦1, x11<x12, x11<x13<x12. One of the Alx13Ga1-x13As slope content layers 112C has a stepwise increasing Al content x13 between x11 and x12 as viewed from the high-reflectivity layer 112A toward the low-reflectivity layer 112B, and another of Alx13Ga1-x13As slope content layers 112C has a stepwise decreasing Al content x13 between x12 and x11 as viewed from the low-reflectivity layer 112C toward the high-reflectivity layer 112A. The pair of DBRs 112 and 117 allow the laser generated in the active layer 114 to lase between the DBRs 112 and 117 and pass through the upper DBR 117 as a laser beam having a desired output power.
The current confinement layer 116 is disposed between the p-type cladding layer 115 and the upper DBR 117, has a non-oxidized area 116A as a current injection area and an oxidized-Al area 116B as a current confinement area, and implements a current injection path for injecting current into the active layer 114 from the p-side electrode 119. The current confinement layer 116 may be formed by replacing the Alx12Ga1-x12As low-reflectivity layer 112B of the upper DBR 17 with an AlAs layer in the vicinity of the active layer 114 and selectively oxidizing the Al content in the AlAs layer.
In the conventional VCSEL device as described above, it is observed that the VCSEL device reduces its optical output power during a continuous laser emission operation thereof. The reduction of the optical output power sometimes results in halt of the laser emission itself and degrades the reliability of the optical data transmission.